Internal hydraulic clamp

ABSTRACT

A device for securing a workpiece to a fixture for performing machining operations includes a tubular, radially expansible mandrel. The mandrel is carried in a passage provided in the fixture and extends into a hole provided in the workpiece. An expander stem is carried within a bore of the mandrel. The stem has an upper end that is tapered for expanding the mandrel. The lower end of the stem is connected to a hydraulic piston. A spring applies an upward force on the mandrel. A hydraulic cylinder moves the stem downwardly, while the spring retains the mandrel in a stationary position until it grips the workpiece. Then, the hydraulic force overcomes the force of the spring to pull the mandrel downward slightly to secure the workpiece to the fixture.

The government has rights in this invention pursuant to contract numberF33657-82-C-2034 awarded by the Department of the Air Force.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to devices for clamping parts to bemachined, and in particular to a hydraulically actuated internal clampfor clamping billets for cutting operation on numerical controlmachines.

2. Description of the Prior Art

In the aircraft industry, many parts are fabricated from large slabs ofaluminum known as billets. An initial step is cutting the billets withmultiple head, numerical controlled, milling machines. The billet isclamped to a fixture below the milling machine head by various means.The clamps usually comprise bolts, straps or other types of mechanicalclamps.

These types of clamps have a disadvantage in that the machine cutter hasto go around the clamp, which is especially a problem in numericalcontrol machining using multiple hand milling machines. Also, the clampsmay have to be moved, removed or added during the machining sequence.Often, the machine operator must physically climb up on the machine bedto do anything with the clamps. This creates a loss in time and ahazardous safety problem. Many clamps and bolts used on machine toolsare different sizes, adding to the confusion of selecting the correctdevice. This causes shutdowns to find lost clamps or replace missingcomponents. Any loose or removable part on a machine tool is a seriousobstacle to production requirements. Exposed surface clamping incurs arisk of a high speed cutter striking the clamp due to machinemalfunction or error, also.

Locking collets have been known for some time. Generally, a lockingcollet comprises a tubular mandrel through which an expander stem iscarried. Moving the expander stem in one direction expands the mandrelto lock the part to the mandrel. Normally, the expander stem is part ofa spindle of the machine. Locking collets of this type have been usedfor machining gears and other parts.

While a locking collet may be acceptable for its particular task, inheavy cutting operations, such as concerned herein, a conventionallocking collet would not have sufficient holddown power. Also, mostlocking collets protrude past the workpiece within which they aresecured, creating an obstacle if attempted to be used with a numericalcontrol milling machine. Moreover, these locking collets do not rigidlysecure a workpiece to a flat supporting surface for cutting operations.

SUMMARY OF THE INVENTION

In this invention, a device is provided for securing a workpiece to asupport surface of a fixture for performing heavy machining operations.The device includes a number of expanding mandrel assemblies for holdingdown a workpiece or billet. Each assembly includes a mandrel whichprotrudes through a passage in the fixture supporting surface and into ahole provided in the workpiece. An expander stem is carried within thebore of the mandrel. The stem has an upper end that protrudes past themandrel and is tapered for expanding the mandrel. The upper end of thestem is preferably recessed within the hole in the workpiece so as toavoid an obstruction. A spring applies an upward force to the mandrel,while a stop cooperating with the mandrel limits the upward movement.

Hydraulic means pulls downwardly on the stem with hydraulic force toexpand the upper end of the mandrel into gripping engagement with theworkpiece. The spring keeps the mandrel in an upper position until thesides of the mandrel can expand out and tightly grip the workpiece. Thenthe spring force is overcome by the hydraulic force, which pulls themandrel down a slight distance to hold the workpiece tightly against thesupporting surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional, partially schematic view of an internal clampconstructed in accordance with this invention.

FIG. 2 is an enlarged view of a portion of the clamp of FIG. 1, andshown in the clamping position.

FIG. 3 is a side elevational view of the mandrel used with the clamp ofFIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, hydraulic clamp 11 includes a fixture 13. Fixture13 is a metal structure having a base 15 and a support surface 17located above base 15. Support surface 17 is a flat plate that isparallel with the horizontal base 15. Support surface 17 is adapted tosupport a workpiece "W" below a machine tool (not shown). Workpiece "W"often will be a solid block or billet of aluminum, into which a numberof parts will be cut. Vertical supports 19, normally cylindricalcolumns, support the support surface 17 above the base 15.

The support surface 17 has a plurality of passages 21 (only one shown)that extend completely through the support surface. Each passage 21 isadapted to align co-axially with a hole 23 formed in the workpiece "W".Hole 23 must extend from the lower side of the workpiece "W", but doesnot necessarily have to extend completely through the workpiece "W".Hole 23 could be a blind hole extending from the lower side of workpiece"W" a selected distance upward.

A sleeve or bushing 25 is stationarily secured within the supportsurface passage 21. Bushing 25 closely and slidingly receives a metalmandrel 27. As shown in FIG'S. 2 and 3, mandrel 27 is a tubular member,having a generally cylindrical wall 29 and an axial bore 31 that extendstherethrough co-axial with the axis of the passage 21. Wall 29 has anenlarged area 29a that is closely received in the bushing 25. Aplurality of slots 33 (FIG. 3), of which there are three in thepreferred embodiment, extend from the upper edge of the mandrel 27 to apoint just above the enlarged area 29a. Each slot 33 extends completelythrough the wall 29 to allow the wall 29 to expand radially outward. Aplurality of serrations or teeth 35 are formed on the exterior of wall29 at the upper end of the mandrel 27. Serrations 35 serve as grippingmeans for gripping the workpiece "W" within the hole 23. A flange 37 isformed on the lower end of mandrel 27. Flange 37 has a diameter largerthan passage 21.

An expander stem 39 extends slightly through the bore 31 of mandrel 27.Stem 39 is a solid rod of metal having a tapered upper end 41, that ispreferably frusto-conical. Upper end 41 is larger in diameter than thebore 31 so that when stem 39 is pulled downwardly, it will cause themandrel 27 to radially expand. Bore 31 is tapered at its upper edge tomate with the taper of upper end 41. Stem 39 has a plurality of threads43 on its lower end. An upper lock nut 45 engages threads 43 andsupports a washer or stop member 47. A coil spring 49 is located betweenthe flange 37 and the washer 47. Rotating nut 45 moves washer 47upwardly to compress the coil spring 49.

A lower nut 55 also engages the threads 43. The threads 43 of stem 39extend into and threadingly engage a protruding portion of a piston 53.Nut 55 locates and locks the stem 39 to the piston 53. Piston 53 has alower enlarged diameter portion (not shown) located within a hydrauliccylinder 57. Hydraulic cylinder 57 receives hydraulic fluid throughlines 59. A pump 61 supplies hydraulic fluid to the hydraulic cylinder57 to move the piston 53 upwardly and downwardly. A control valve 63controls the direction of the hydraulic fluid flow. A pressure reliefvalve 65 limits the hydraulic pressure.

In assembling the hydraulic clamp 11, the hydraulic cylinder 57 will bemounted to base 15. Bushing 25, mandrel 27, stem 39 and the variouscomponents will be assembled as shown in FIG. 2. Hydraulic power issupplied to hydraulic cylinder 57 to move piston 53 to the top of itsstroke. Stem 39 will be screwed into the piston 53 and upper lock nut 45tightened to a position in which the upper edge of the mandrel 27engages the lower point of the tapered end 41, and the upper side offlange 37 touches the lower side of support surface 17. The positionwill be selected such that any downward movement of stem 39 with respectto mandrel 27 will begin to expand the mandrel 27. Lower lock nut 55will be secured to maintain the stem 39 at this position.

The upper lock nut 45 is then further tightened to compress the spring49 to a desired preload. The upward force of the spring 49 on flange 37of mandrel 27 must be sufficient to keep the mandrel from movingdownward when the stem 39 starts to move downward, until the mandrel 27has expanded into engagement with the workpiece "W". When preloaded tothe desired amount, stem 39 will move downwardly slightly, and alongwith it piston 53. This causes mandrel 27 to expand radially a slightamount before hydraulic force is applied to move piston 53 downwardly.

In operation, first, a plurality of holes 23 are drilled in theworkpiece "W", one hole 23 for each expander stem 39 and mandrel 27assembly located in fixture 13. The workpiece "W" then is placed on thesupport surface 17 of the fixture 13. The mandrel serrations 35 will bespaced slightly inward from the wall of the hole 23 initially. Mandrel27 will have an upward force imposed upon it by the spring means or coilspring 49. The upward travel of mandrel 27 will be limited by the stopmeans comprising the upper surface of the flange 37 in contact with thelower surface of support surface 17. Then, hydraulic pressure issupplied through lines 59 to the upper side of piston 53 in hydrauliccylinder 57. This causes the piston 53 to move downwardly. Initially,mandrel 27 will not move downwardly because of the force of the spring49.

The movement of stem 39 with respect to mandrel 27 causes the upper endof the mandrel to radially expand, with serrations 35 biting into thewall of hole 23 in workpiece "W". The lower end of the spring 49 movesdownwardly in unison with stem 39 because washer 47 is rigidly securedto the stem 39. The upward force of the spring 49 begins to decrease asthe spring elongates due to downward movement of the stem 39.Eventually, the hydraulic force pulling downward on stem 39 will exceedthe declining upward force imposed by the spring 49 on mandrel flange37, causing mandrel 27 to also move downwardly until a selectedhydraulic pressure is reached. This downward movement of mandrel 27 doesnot occur, however, until the serrations 35 have tightly gripped theworkpiece "W". Workpiece "W" does not move downward with mandrel 27. Theserrations 35 deform the wall of hole 23 slightly. The downward movementof mandrel 27 is slight, about 0.010 inch in the preferred embodimentfor an aluminum workpiece "W". This results in a clearance 67 betweenthe upper edge of flange 37 and the lower side of the fixture supportsurface 17. The downward travel of mandrel 27 is completelyunobstructed, and stops only when the hydraulic cylinder 57 reaches theselected maximum pressure set by pressure relief valve 65. The selectedmaximum hydraulic pressure on stem 39 is maintained during machining andwill be transmitted to the mandrel 27, which transmits the force to theworkpiece "W", to tightly secure the workpiece to the support surface17. The downward travel of stem 39 and stop washer 47 is indicated inFIG. 2 by the symbol "y" and by the phantom lines.

Hydraulic force will be maintained on the stem 39 until the machiningoperations are finished on the workpiece "W". Then, the control valve 63is switched to the opposite position, applying hydraulic pressure to thelower side of piston 53 to cause the piston 53 to move upwardly. As thestem 39 moves upwardly, the resiliency of mandrel 27 causes its wall 29to move inwardly to return to the unlocked position. The workpiece "W"can be moved simply by lifting upward. Once in the upper position, asshown in FIG. 1, the preload force on the spring 49 will again return tothe selected amount.

In the preferred embodiment, the hole 23 in workpiece "W" is preferablyone inch. The outer diameter of mandrel 27 at serrations 35 is initiallyabout 0.960 inch. The preload force on coil spring 49 is preferably 800pounds and this causes the stem 39 to move downwardly slightly,expanding the mandrel 27 to a diameter at serrations 35 of about 0.980inch. The taper of upper end 41 is 20 degrees. The distance "y" varieswith the maximum hydraulic pressure and the workpiece "W". The preferredmaximum hydraulic pressure is 500 psi (pounds per square inch) and themaximum piston stroke is preferably 0.250 inch. The piston 53 willnormally not move downwardly more than two-thirds of its stroke beforethe maximum hydraulic pressure is reached.

The invention has significant advantages. The hydraulic clamp is fastand simple to use. In multiples, the clamp provides sufficient holddownforce for heavy cutting operations. The clamp does not protrude abovethe surface of the workpiece. The same clamp can be used with manydifferent types of machining operations.

While the invention has been shown in only one of its forms, it shouldbe apparent to those skilled in the art that it is not so limited but issusceptible to various changes without departing from the scope of theinvention.

I claim:
 1. A device for securing a workpiece to a support surface of afixture for performing machining operations, comprising:a mandreladapted to be carried in a passage provided in the fixture supportsurface and having an upper end that is expansible and is adapted toextend into a hole provided in the workpiece, the mandrel having aflange on its lower end that is larger in diameter than the passage; anexpander stem carried within a bore of the mandrel, having an upper endthat protrudes past the upper end of the mandrel and is adapted to berecessed within the hole in the workpiece, the upper end of the stembeing tapered and larger in diameter than the bore of the mandrel;hydraulic means for pulling downwardly on the stem with hydraulic force;and spring means for applying an upward force on the mandrel, causingthe flange to bear against the lower side of the support surface whilethe stem is in an upper position, to initially prevent downward movementof the mandrel to expand the upper end of the mandrel into grippingengagement with the workpiece as the stem moves downwardly, thehydraulic force subsequently pulling the mandrel downwardly with thestem after the mandrel grips the workpiece, for tightly securing theworkpiece to the support surface, the device being free of any structurebelow the flange for stopping movement of the mandrel once the stembegins pulling the mandrel downwardly.
 2. The device according to claim1 wherein the mandrel is free to move downwardly once the force of thespring is overcome, until a selected hydraulic pressure is reached. 3.The device according to claim 1 wherein the mandrel has serrationsformed on its upper end for gripping the workpiece.
 4. The deviceaccording to claim 1 further comprising a bushing encircling the mandrelwithin the passage in the fixture support surface.
 5. A device forsecuring a member to a support surface, comprising:a tubular radiallyexpansible mandrel adapted to be carried in a passage provided in thesupport surface and adapted to extend into a hole provided in themember, the mandrel having a flange on its lower end of diameter largerthan the passage diameter to limit upward movement of the mandrel; anexpander stem carried within a bore of the mandrel, having an upper endthat protrudes past the mandrel, the upper end of the stem being taperedand larger in diameter than the bore of the mandrel; spring means forapplying an upward force on the mandrel, causing the flange of themandrel to bear against the lower side of the support surface while thestem is in an upper position, the spring means including a coil springhaving an upper end that engages the flange of the mandrel and a lowerend that engages a stop carried by the stem for movement therewith, thestop being positioned to compress the spring and provide an upwardpreload force on the mandrel prior to engagement of the member by themandrel; and hydraulic means for pulling the stem downwardly withhydraulic force, the preload force being selected to cause the mandrelto remain axially stationary as the stem and lower end of the springbegin to move downwardly, until the mandrel grips the member, at whichpoint the mandrel moves downwardly to pull the member into tight contactwith the support surface until a selected hydraulic force is reached,the device being free of any structure below the flange for stoppingmovement of the mandrel once the stem begins pulling the mandreldownwardly.
 6. The device according to claim 5 wherein the stop on thestem is adjustable axially relative to the stem for varying the preloadforce of the spring.
 7. A device for securing a workpiece for performingmachining operations, comprising in combination:a fixture having a baseand a support surface spaced apart and separated by supporting members,the support surface having at least one passage therethrough, which hasan axis adapted to align with a hole provided in a workpiece; a tubular,metal mandrel having a plurality of vertical slots in an upper endthereof to allow expansion, the upper end protruding through the passageand having an exterior surface at the upper end with serrations formedthereon for gripping a sidewall of the hole in the workpiece, themandrel having a flange on its lower end that is larger in diameter thanthe passage; an expander stem carried within a bore of the mandrel,having an upper end that protrudes past the upper end of the mandrel andis adapted to be recessed within the hole in the workpiece, the upperend of the stem being tapered and larger in diameter than the bore ofthe mandrel, the stem having a threaded portion located below the upperend; a stop washer supported by a nut that engages the threaded portion,the distance from the stop washer to the supporting surface beingadjustable by rotating the nut; a coil spring encircling the stem andcompressed between the mandrel and the washer to a selected preloadforce that urges the flange of the mandrel against the lower side of thesupport surface while the expander stem is in an upper position; and ahydraulic piston and cylinder mounted on the base and connected to thelower end of the stem for moving the stem downwardly, the preload forcebeing selected to cause the mandrel to remain axially stationary as thestem begins to move downwardly until the serrations begin gripping theworkpiece, the mandrel then moving downwardly free of obstructions untila selected hydraulic force is reached and a slight clearance existsbetween the flange of the mandrel and the lower side of the supportsurface, the device being free of any structure below the flange forstopping movement of the mandrel once the stem begins pulling themandrel downwardly.
 8. The device according to claim 7 furthercomprising a bushing encircling the mandrel within the passage in thefixture support surface.